Ionomers, polymers containing less than 10 mol percent covalently bonded ionic substituents, exhibit considerably higher moduli and higher glass transition temperatures compared to those of their non-ionic analogues. The improvement in the polymer's mechanical and thermal performance is generally attributed to the formation of ionic aggregates. These aggregates act as thermoreversible cross-links and effectively retard the translational mobility of polymeric chains. The thermoreversible nature of the ionic aggregation may address many other disadvantages associated with covalently bonded high molecular weight polymers, such as poor melt processability, high melt viscosity, and low thermal stability at typical processing conditions such as high shear rate and temperature.
Telechelic ionomers are polymers containing the ionic substituents located only at the chain ends. Such ionomers exhibit ionic aggregation only at the end of the chain, giving rise to an electrostatic chain extension unlike random ionomers that tend to give rise to a gel-like or cross linked aggregation.
Sulfonated telechelic ionomeric poly(ethylene terephthalate) (PET) is a known telechelic ionomer. Two methods are known for the synthesis of PET sulfonated telechelic ionomers: i) reactive blending of PET with aliphatic esters of sulfobenzoic acid (Berti et al., Macromol. Symp. 2001, 176, 211) and ii) addition of sulfobenzoic acid to the melt polymerization process of ethylene glycol and dimethyl terephthalate (Kang et al., Macromolecules, 2002, 23, 8738). Both methods have their disadvantages, especially for the preparation of higher molecular weight telechelic ionomers. For example, in the first method the reaction of the sulfobenzoic acid esters with PET polymer gives rise to a chain scission resulting in PET ionomers of lower molecular weight than the starting polymer. In the second method, the sulfobenzoic acid has a high melting temperature and low solubility in the polymer melt. These properties of the sulfobenzoic acid result in the need for polar monomers, high reaction temperatures, and long residence times in order to obtain the ester exchange reaction to produce the ionomeric polyester product.
There is therefore a need for an improved method of preparing telechelic ionomeric polyesters, as well as a method of preparing high molecular weight telechelic ionomeric polyesters.